1. Field of the Invention
The present invention relates to a combustion control system and a combustion control method for a direct fuel injection spark ignition engine. More particularly, the present invention relates to a direct fuel injection combustion control system for a direct fuel injection spark ignition engine having a high-pressure fuel delivery system configured to supply fuel to a fuel injection valve that injects fuel directly into a combustion chamber and maintain a constant pressure inside an accumulator when fuel is not being injected, the direct fuel injection spark ignition engine being controlled such that fuel injection from the fuel injection valve is temporarily stopped under prescribed operating conditions and resumed after a prescribed amount of time has elapsed.
2. Background Information
The fuel efficiency of direct fuel injection spark ignition engines is being improved by injecting fuel directly into the combustion chamber during the compression stroke, generating a very rich air-fuel mixture in the vicinity of the spark plug, and performing stratified combustion so that, overall, a lean air-fuel mixture is be used. Since the fuel is injected against the high pressure of the combustion chamber during the compression stroke, it is necessary to use a fuel pump system that injects the fuel with higher pressure than is used in port injection engines. This fuel pump system is generally an accumulator-type system in which fuel is pumped into an accumulator having a constant volume with a high-pressure fuel pump. As the fuel is injected into the combustion chamber with a fuel injection valve, the fuel pressure inside the accumulator is held at a target pressure by using a fuel pressure sensor and replenishing the fuel. The target fuel pressure required varies depending on such operating conditions as the engine rotational speed and the load. Normally, target fuel pressures corresponding to different engine speeds and loads are set in advance. In an accumulator-type fuel pump system, the fuel pressure inside the accumulator remains constant so long as fuel is not injected from the fuel injection valve.
This kind of direct fuel injection spark ignition engine generally performs fuel cutting, i.e., stops injection of the fuel, when it is not necessary to generate torque, such as when the accelerator is released during deceleration or after engine warm-up. After the fuel injection is stopped, the target fuel pressure decreases along with the decrease in engine rotational speed and load but the actual fuel pressure inside the accumulator remains at the fuel pressure that existed at the point in time when fuel injection was stopped. Consequently, a gap exists between the actual fuel pressure and the target fuel pressure when it is time to resume fuel injection and the penetration of the fuel spray will not be appropriate if fuel injection is resumed at the fuel injection timing and ignition timing corresponding to the target fuel pressure. More specifically, the fuel injection timing and ignition timing are too early in view of the actual fuel pressure in the accumulator. Consequently, the air-fuel mixture having the optimum air-fuel ratio will not be at the desired location when spark ignition occurs.
As a result, there is a possibility that any of the following may occur: the operating performance will degrade when the vehicle accelerates due to the resumption of fuel injection; degraded combustion will cause incompletely burned gasses to be discharged due to misfiring, leading to degraded exhaust gas; and the fuel cannot be ignited and the engine will stall.
One existing method for addressing this problem is to establish a delay time for allowing the fuel pressure to decrease between the time when the fuel injection stop conditions are satisfied and the time when the fuel injection is stopped. The fuel injection is continued for the duration of the delay time and the fuel pressure inside the accumulator is reduced to the target fuel pressure. For example, see Japanese Laid-Open Patent Publication No. 2000-18067 (pages 3 and 4, FIG. 5).
Another method is to modify the injection pulse width in accordance with the delay in the rise of the fuel pressure, thereby preventing the discharge of unburned gases. For example, see Japanese Unexamined Utility Model Publication No. 3-119551.
Still another method is to calculate the difference between the pressure inside the combustion chamber and the fuel pressure inside the accumulator and modify the injection pulse time based on that difference, thereby allowing the desired fuel injection quantity to be obtained even the injection timing is changed. For example, see Japanese Laid-Open Patent Publication No. 9-228864 (page 4, FIG. 3)
In view of the above, it will be apparent to those skilled in the art from this disclosure that there exists a need for an improved direct fuel injection combustion control system. This invention addresses this need in the art as well as other needs, which will become apparent to those skilled in the art from this disclosure.
It has been discovered that in the technology of Japanese Laid-Open Patent Publication No. 2000-18067 a long delay time (e.g., 1000 msec) is required to be set in order to lower the actual fuel pressure to the target fuel pressure. Since fuel continues to be injected during this period, the act of stopping the fuel injection does not provide the effect of reducing the fuel consumption.
The technology of Japanese Unexamined Utility Model Publication No. 3-119551 adjusts the injection pulse width, but the operating performance at the time of fuel injection resumption cannot be sufficiently improved by merely adjusting the injection pulse width. The technology of Japanese Laid-Open Patent Publication No. 9-228864 detects the pressure inside the combustion chamber and the fuel pressure inside the accumulator and adjusts the injection pulse width based on the detected pressure values, but, again, the operating performance at the time of fuel injection resumption cannot be sufficiently improved.
The present invention concerns combustion control systems for direct fuel injection spark ignition engines configured to maintain a constant pressure in the accumulator when fuel is not being injected. An object of the present invention is to improve the combustion control executed by such a combustion control system when fuel injection is resumed after being temporarily stopped.
The present invention basically relates a combustion control system for a direct fuel injection spark ignition engine that comprises a target fuel pressure acquiring section, an actual fuel pressure detecting section, a timing acquiring section, and a first timing control section. The target fuel pressure acquiring section is configured to acquire a target fuel pressure required to resume fuel injection based on operating conditions existing when fuel injection is determined to be resumed after a prescribed amount of time has elapsed since fuel injection was temporarily stopped due to occurrence of prescribed operating conditions. The actual fuel pressure detecting section is configured to detect an actual fuel pressure inside an accumulator of a high-pressure fuel delivery system when fuel injection is determined to be resumed. The timing acquiring section is configured to acquire a fuel injection timing and an ignition timing required to resume fuel injection based on the operating conditions existing when fuel injection is determined to be resumed. The first timing control section is configured to compare the target fuel pressure and the actual fuel pressure and modify at least one of the fuel injection timing and the ignition timing based on a result of the comparison when fuel injection is to be resumed.
These and other objects, features, aspects and advantages of the present invention will become apparent to those skilled in the art from the following detailed description, which, taken in conjunction with the annexed drawings, discloses a preferred embodiment of the present invention.